Rather than diagnosing/replacing the chipped extractor, bad spring, improperly staked key, etc in an AR, I simply pull out the whole BCG, and slap in a brand new BCG from my parts kit and rock on. I can fiddle with the "why" later, and not screw up my day, match, class etc.
Same with Glocks.
Besides, if suddenly there is a run on parts due to changes in political winds, I have peace of mind.